Inhibitors of the carrier-mediated influx of auxin in suspension-cultured tobacco cells

Abstract

Active auxin transport in plant cells is catalyzed by two carriers working in opposite directions at the plasma membrane, the influx and efflux carriers. A role for the efflux carrier in polar auxin transport (PAT) in plants has been shown from studies using phytotropins. Phytotropins have been invaluable in demonstrating that PAT is essential to ensure polarized and coordinated growth and to provide plants with the capacity to respond to environmental stimuli. However, the function of the influx carrier at the whole-plant level is unknown. Our work aims to identify new auxin-transport inhibitors which could be employed to investigate its function. Thirty-five aryl and aryloxyalkylcarboxylic acids were assayed for their ability to perturb the accumulation of 2,4-dichlorophenoxyacetic acid (2,4-D) and naphthalene-1-acetic acid (1-NAA) in suspension-cultured tobacco (Nicotiana tabacum L.) cells. As 2,4-D and 1-NAA are preferentially transported by the influx and efflux carriers, respectively, accumulation experiments utilizing synthetic auxins provide independant information on the activities of both carriers. The majority (60%) of compounds half-inhibited the carrier-mediated influx of [14C]2,4-D at concentrations of less than 10 microM. Most failed to interfere with [3H]NAA efflux, at least in the short term. Even though they increasingly perturbed auxin efflux when given a prolonged treatment, several compounds were much better at discriminating between influx and efflux carrier activities than naphthalene-2-acetic acid which is commonly employed to investigate influx-carrier properties. Structure-activity relationships and factors influencing ligand specificity with regard to auxin carriers are discussed.

Auxin transport and activity regulate stomatal patterning and development

“…Pattern violations also appeared when auxin efflux was blocked chemically with 1-naphthylphthalamic acid (NPA) 27 . By contrast, patterning was unaffected by the application of native or synthetic auxin (IAA, NAA and 2,4-dichlorophenoxyacetic acid) or the auxin influx inhibitor (1-naphthoxyacetic acid) 28 , suggesting that intracellular auxin pools are tightly and locally regulated (Fig. 2d,e).…”

The Binding of Auxin to the Arabidopsis Auxin Influx Transporter AUX1

“…In the current study, two synthetic auxins and three auxin influx inhibitors (Imhoff et al, 2000;Parry et al, 2001) were analyzed for their ability to displace IAA binding from AUX1. All were able to completely displace the bound IAA, and the IC 50 values for this displacement can be compared with their relative abilities to inhibit radiolabeled 2,4-D uptake into tobacco (Nicotiana tabacum) suspension cells (Delbarre et al, 1996;Imhoff et al, 2000).…”

Mutations in an Auxin Receptor Homolog AFB5 and in SGT1b Confer Resistance to Synthetic Picolinate Auxins and Not to 2,4-Dichlorophenoxyacetic Acid or Indole-3-Acetic Acid in Arabidopsis

“…This suggests that some degree of selectivity may occur at or close to the target site. There is evidence for differential cellular uptake of synthetic auxins by auxin permeases (Delbarre et al, 1996;Imhoff et al, 2000;Hoessel et al, 2005) that could account for some differences in herbicidal efficacy between different auxins. Our present data suggest that some degree of synthetic auxin selectivity and potency may also lie within components of the SCF ubiquitin ligase machinery, including the F-box protein receptors.…”